The present invention relates to linear or multidimensional polyester oligomers and blends that include unsaturated hydrocarbon end caps to provide solvent resistance through crosslinking upon curing. Preferably the backbone includes electronegative (xe2x80x9csulfonexe2x80x9d) segments that alternate with the ester linkages.
Recently, chemists have sought to synthesize oligomers for high performance advanced composites suitable for aerospace applications. These composites should exhibit solvent resistance, be tough, impact resistant, and strength, be easy to process, and be thermoplastic. Oligomers and composites that have thermo-oxidative stability, and, accordingly can be used at elevated temperatures, are particularly desirable.
While epoxy-based composites are suitable for many applications, their brittle nature and susceptibility to degradation make them inadequate for many aerospace applications, especially those applications which require thermally stable, tough composites. Accordingly, research has recently focused upon polyimide composites to achieve an acceptable balance between thermal stability, solvent resistance, and toughness. The maximum use temperatures of conventional polyimide composites, such as PMR-15, are still only about 600-625xc2x0 F., since they have glass transition temperatures of about 690xc2x0 F.
Linear polysulfone, polyether sulfone, polyester, and polyamide systems are also known, but each of these systems fails to provide as high thermal stability as is required in some aerospace applications.
There has been a progression of polyimide sulfone compounds synthesized to provide unique properties or combinations of properties. For example, Kwiatkowski and Brode synthesized maleic-capped, linear polyarylimides as disclosed in U.S. Pat. No. 3,839,287. Holub and Evans synthesized maleic- or nadic-capped, imido-substituted polyester compositions as disclosed in U.S. Pat. No. 3,729,446. We synthesized thermally stable polysulfone oligomers as disclosed in U.S. Pat. No. 4,476,184 or U.S. Pat. No. 4,536,559, and have continued to make advances with polyetherimidesulfones, polybenzoxazolesulfones (i.e., heterocycles), polybutadienesulfones, and xe2x80x9cstarxe2x80x9d or xe2x80x9cstar-burstxe2x80x9d multidimensional oligomers. We have shown surprisingly high glass transition temperatures and desirable physical properties in many of these oligomers and their composites, without losing ease of processing.
Multidimensional oligomers, such as disclosed in our U.S. Pat. No. 5,210,213; are easier to process than many other advanced composite oligomers since they can be handled at lower temperatures. Upon curing, however, the unsaturated phenylimide end caps crosslink so that the thermal resistance of the resulting composite is markedly increased with only a minor loss of stiffness, matrix stress transfer (impact resistance), toughness, elasticity, and other mechanical properties. Glass transition temperatures above 950xc2x0 F. are achievable.
Commercial polyesters, when combined-with well-known diluents, such as styrene, do not exhibit satisfactory thermal and oxidative resistance to be useful for aircraft or aerospace applications. Polyarylesters are unsatisfactory, also, since the resins often are semicrystalline which makes them insoluble in laminating solvents, intractable in fusion, and subject to shrinking or warping during composite fabrication. Those polyarylesters that are soluble in conventional laminating solvents remain so in composite form, thereby limiting their usefulness in structural composites. The high concentration of ester groups contributes to resin strength and tenacity, but also makes the resin susceptible to the damaging effects of water absorption. High moisture absorption by commercial polyesters can lead to distortion of the composite when it is loaded at elevated temperature.
High performance, aerospace, polyester advanced composites, however, can be prepared using crosslinkable, end-capped polyester imide ether sulfone oligomers that have an acceptable combination of solvent resistance, toughness, impact resistance, strength, ease of processing, formability, and thermal resistance. By including Schiff base (xe2x80x94CHxe2x95x90Nxe2x80x94), imidazole, thiazole, or oxazole linkages in the oligomer chain, the linear, advanced composites formed with polyester oligomers of our copending application U.S. Ser. No. 726, 259 now abandoned, can have semiconductive or conductive properties when appropriately doped.
Conductive and semiconductive plastics have been extensively studied (see, e.g., U.S. Pat. Nos. 4,375,427; 4,338,222; 3,966,987; 4,344,869; and 4,344,870), but these polymers do not possess the blend of properties which are essential for aerospace applications. That is, the conductive polymers do not possess the blend of (1) toughness, (2) stiffness, (3) elasticity, (4) ease of processing, (5) impact resistance (and other matrix stress transfer capabilities), (6) retention of properties (over a broad range of temperatures), and (7) high temperature resistance that is desirable on aerospace advanced composites. These prior art composites are often too brittle.
Thermally stable multidimensional oligomers having semiconductive or conductive properties when doped with suitable dopants are also known and are described in our copending applications (including U.S. Ser. No. 773,381 now U.S. Pat. No. 5,968.640 to Lubowitz, Sheppard, and Torre). The linear arms of the oligomers contain conductive linkages, such as Schiff base (xe2x80x94Nxe2x95x90CHxe2x80x94) linkages, between aromatic groups. Sulfone and ether linkages are interspersed in the arms. Each arm is terminated with a mono- or difunctional end cap (i.e., a radical having one or two crosslinking sites) to allow controlled crosslinking upon heat-induced or chemically-induced curing.
Polyamides of this same general type are described in U.S. Pat. No. 4,876,328; polyetherimides, in U.S Pat. No. 5,104,965 and polyamideimides, in U.S. Pat. No. 5,164,967.
High performance, aerospace advanced composites can be prepared using crosslinkable, end-capped polyester sulfone oligomers of the present invention that have a desired blend of solvent resistance, toughness, impact resistance, strength, ease of processing, formability, and thermal resistance. The linear polyester oligomers include compounds selected from the group consisting of: 
and are prepared by the condensation of acid halides (or acids) and dialcohols (bisphenols), wherein:
E=a crosslinkable (unsaturated) end cap to improve the solvent resistance of the cured oligomer in the advanced composite;
A=a residue of a dicarboxylic acid halide; and
B=a residue of a dialcohol.
Generally, A and B are each linear aromatic moieties that include electronegative (xe2x80x9csulfonexe2x80x9d) linkages between aromatic radicals. Suitable electronegative linkages are selected from xe2x80x94SO2xe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94Oxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94(CH3)2Cxe2x80x94, and xe2x80x94(CF3)2Cxe2x80x94. Schiff base (xe2x80x94CHxe2x95x90Nxe2x80x94), oxazole, thiazole, and imidazole linkages can also be used in the backbone where semiconductive or conductive advanced composites are desired, since these linkages are conductive when suitably doped. Preferred Schiff base oligomers have the general formula: 
wherein
R=an aromatic moiety or a short aryl chain including a plurality of aryl moieties linked with any of xe2x80x94CH2xe2x80x94, xe2x80x94SO2xe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94Oxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94(CH3)2Cxe2x80x94, or xe2x80x94(CF3)2Cxe2x80x94; and q=0-4.
The crosslinkable end caps (E) are usually imidophenols or phenylimide acid halides that include a radical selected from the group consisting of: 
wherein
R1=lower alkyl, lower alkoxy, aryl, aryloxy, substituted alkyl, substituted aryl, halogen, or mixtures thereof;
j=0, 1, or 2;
G=xe2x80x94CH2xe2x80x94, xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, or xe2x80x94SO2xe2x80x94;
T=methallyl or allyl; and
Me=methyl.
Multidimensional oligomers include an aromatic hub and three or more arms wherein each arm includes an ester linkage and an end cap.
Blends of the linear or multidimensional oligomers comprise mixtures of the oligomers and one or more compatible, noncrosslinking polymers, and may be interpenetrating networks (IPNs).
The oligomers and blends can be processed with suitable reinforcing additives in continuous or chopped form to make prepregs. Upon curing, the composites are relatively thermally stable, tough, and solvent resistant.
The oligomers are prepared by simultaneously condensing the end caps, dicarboxylic acid halides, and dialcohols, (and hubs, if making multidimensional oligomers) in a suitable solvent under an inert atmosphere.